1. Field of the invention
The present invention relates to a life-size imaging rod lens array comprising a plurality of small-diameter rod lenses arranged in two rows. The invention also relates to a life-size imaging optical apparatus which has the surface of a document and a sensor provided on opposite sides of the rod lens array such that the image on the surface of a document is read and transmitted to the sensor. The technology of the invention is useful when applied to the optics of facsimile and other optical machines.
2. Related Art
Optical machines such as facsimile, copiers, printers and scanners employ various types of scanning apparatus in order to convert the information on the surface of a document to readable electrical signals. A common scanning apparatus is of a "contact" type that comprises illumination optics, a rod lens array as life-size imaging optics, a sensor, a cover glass (transparent substrate) and other parts mounted in a frame. The document is placed in contact with the surface of the cover glass and illuminated with light from the illumination optics. The illuminated image on the document passes through the rod lens array to be focused on the sensor, where it is converted to an electrical signal. The rod lens array is life-size imaging optics in which a plurality of rod lenses each having a refractive-index distribution in the radial direction are arranged, typically in one or two rows.
The lens material of which the rod lens array is made is either glass or plastics. Glass rod lenses with a refractive-index distribution are manufactured by ion-exchange or thermal interdiffusion or a like method. For example, the rod lenses in a commercial glass rod lens array have a minimum outside diameter of 0.6 mm.
A single rod lens forms a life-size image in a range defined by a circle with a radius of X.sub.0 (i.e., the view radius) and the quantity of light which is maximal on the optical axis decrease with the increasing radial distance. Therefore, the distribution of the quantity of light along the length of the lens array experiences unevenness at a period which is equal to the distance between adjacent lenses. The magnitude of the unevenness is determined by the degree of overlap m which is defined by m=X.sub.0 /2R, where 2R is the distance between the optical axes of adjacent rod lenses.
FIG. 1 shows how the unevenness in the quantity of light varies as a function of the degree of overlap m in a two-row rod lens array for a so-called "line scanning system" which utilizes light in an extremely narrow range near to the overall optical axis of the lens array. Obviously, the unevenness in the quantity of light tends to decrease with the increasing degree of overlap m; however, the decrease is not monotonic but minima occur at m=0.91, 1.13, 1.37, 1.61, 1.85, . . . The smaller the amount of unevenness in the quantity of light on the sensor, the better its performance; hence, if it is paramount to minimize the unevenness in the quantity of light, the lens array is designed to ensure that the degree of overlap m assumes the above-mentioned values.
The values of unevenness in the quantity of light that are shown in FIG. 1 are those which occur when the sensor is positioned accurately on the overall optical axis of the lens array. However, actual mass-produced scanning apparatus unavoidably suffer from a certain amount of transverse misalignment between the sensor and the overall axis of the lens array on account of dimensional errors in parts and assembling errors.
In the conventional lens array in which each rod lens has an outside diameter of 0.6 mm .phi. or more, the above-defined transverse misalignment is sufficiently smaller than the view radius X.sub.0 that it can be ignored without any problem. However, the idea of reducing the rod lens diameter is recently under review in order to realize a compact scanning apparatus and if rod lenses with a diameter smaller than 0.5 mm .phi. are employed, the relative amount of transverse misalignment between the sensor and the overall axis of the lens array increases to potentially cause an unevenness in the quantity of light that exceeds the design value.